The present invention relates generally to improved methods for the microbial production of acetic acid. More particularly, the invention relates to extraction of acetic acid from aqueous streams, and from the microbial fermentation of desirable chemical products from gaseous streams, such as waste gas streams, industrial gas streams, or from gas streams produced from the gasification of carbonaceous materials.
Methods for the anaerobic fermentation of carbon monoxide, and/or hydrogen and carbon dioxide to produce acetic acid, acetate salts or other products of commercial interest, such as ethanol, have been performed at laboratory bench scale. See, e.g., Vega et al, (1989) Biotech. Bioeng., 34:785–793; Klasson et al (1990) Appl. Biochem. Biotech., 24/25: 1; Vega et al (1989) Appl. Biochem. Biotech., 20/21: 781–797; and Klasson et al (1992) Enz. Microbio. Tech., 19:602–608, among others. More recently, the present inventors have discussed large-scale methods for the fermentation of industrial gas streams, particularly waste gas streams, into products of commercial use by using methods employing fermentation of the gas stream, an aqueous nutrient medium and an anaerobic bacteria or mixtures thereof in a bioreactor. See, e.g., U.S. Pat. No. 5,173,429; U.S. Pat. No. 5,593,886 and International Patent Publication No. WO98/00558, incorporated herein by reference.
According to the above-cited prior art of the inventors, one such large scale process involves the following summarized steps. Nutrients are continuously fed to a bioreactor or fermenter in which resides a culture, either single or mixed species, of anaerobic bacteria. A gas stream is continuously introduced into the bioreactor and retained in the bioreactor for sufficient time to maximize efficiency of the process. Exhaust gas containing inert and unreacted substrate gases, are then released. The liquid effluent is passed to a centrifuge, hollow fiber membrane, or other solid-liquid separation device to separate out microorganisms that are entrained. These microorganisms are returned to the bioreactor to maintain a high cell concentration which yields a faster reaction rate. Separation of the desired biologically produced product(s) from the permeate or centrifugate occurs by passing the permeate or centrifugate to an extractor where it is contacted with a solvent, such as a di-alkyl and tri-alkyl amine in a suitable cosolvent, or tributyl phosphate, ethyl acetate, tri-octyl phosphine oxide and related compounds in a co-solvent. Suitable cosolvents include long chain alcohols, hexane, cyclohexane, chloroform, and tetrachloroethylene.
The nutrients and materials in the aqueous phase pass back to the bioreactor and the solvent/acid/water solution passes to a distillation column, where this solution is heated to a sufficient temperature to separate the acid and water from the solvent. The solvent passes from the distillation column through a cooling chamber to lower the temperature to the optimum temperature for extraction, then back to the extractor for reuse. The acid and water solution passes to a final distillation column where the desired end product is separated from the water and removed. The water is recirculated for nutrient preparation.
Further, a variety of acetogenic bacteria are well known to produce acetic acid and other commercially interesting products when subjected to such fermentation processes, including novel strains of Clostridium ljungdahlii[See, e.g., U.S. Pat. Nos. 5,173,429 and 5,593,886 and International Patent Publication No. WO98/00558].
Despite such knowledge and advances in the art of microbial fermentation of a variety of gas streams, acetic acid production is limited by the acetic acid loading potential of the solvent used, and by the degradation of the solvent as it travels through the production process, among other issues. In view of the ever-increasing need to produce acetic acid, as well as to convert industrial waste gases into useful non-polluting products, there remains a need in the art for processes which are more efficient in producing the desired commercial product and compositions which can enhance performance of such methods.